According to the graph, what is the average monthly balance, in dollars, of David's savings account during the four-month period shown? [asy] draw((0,0)--(13,0)--(13,8)--(0,8)--cycle,linewidth(1)); draw((0,2)--(13,2),linewidth(1)); draw((0,4)--(13,4),linewidth(1)); draw((0,6)--(13,6),linewidth(1)); draw((1,0)--(1,2)--(3,2)--(3,0)--cycle, linewidth(1)); fill((1,0)--(1,2)--(3,2)--(3,0)--cycle,lightgray); draw((4,0)--(4,4)--(6,4)--(6,0)--cycle, linewidth(1)); fill((4,0)--(4,4)--(6,4)--(6,0)--cycle,lightgray); draw((7,0)--(7,3)--(9,3)--(9,0)--cycle, linewidth(1)); fill((7,0)--(7,3)--(9,3)--(9,0)--cycle,lightgray); draw((10,0)--(10,3)--(12,3)--(12,0)--cycle, linewidth(1)); fill((10,0)--(10,3)--(12,3)--(12,0)--cycle,lightgray); label("J",(2,0),S); label("F",(5,0),S); label("M",(8,0),S); label("A",(11,0),S); label("100",(2,2),S); label("200",(5,4),S); label("150",(8,3),S); label("150",(11,3),S); label("100",(0,2),W); label("200",(0,4),W); label("300",(0,6),W); label(rotate(90)*Label("Account Balance",(-4,4))); label(rotate(90)*Label("(in dollars)",(-3,4))); label("David's Savings Account Balance",(6,11)); label("Month",(6,-3)); [/asy]
To find the average, we take the total sum and divide by the number of terms: $\frac{100+200+150+150}{4}=\frac{600}{4}=150$. The average monthly balance is $\boxed{\$150}$.